1. Field of the Invention
The present invention relates to an organic photoconductive material and an electrophotographic photoreceptor to which the organic photoconductive material is applied. More specifically, the present invention relates to a novel organic photoconductive material suitable as a charge-transporting material in a photosensitive layer of an electrophotographic photoreceptor or usable in various display devices, and an electrophotographic photoreceptor which uses the above organic photoconductive material in a photosensitive layer, which exhibits a high charge potential and high sensitivity, and which exhibits little or almost no change in various properties in repeated use and can exhibit stabilized performances.
2. Explanation of Related Art
In recent years, the use of electrophotography is not limited to the field of a copying machine, and is widely applied to conventionally photography-applied fields of a printing plate material, a slide film, a microfilm, and the like. Further, studies are being made for the application thereof to a high-speed printer using a laser, LED or CRT as a light source. Further, studies are being made for applications of photoconductive materials to uses other than an electrophotographic photoreceptor, for example, various display devices such as a sensor material, an electrostatic recording device, an EL device, an optical switching element and an electronic paper. Accordingly, there are increasing sophisticated broad demands for a photoconductive material and an electrophotographic photoreceptor using such a photoconductive material. As a conventional electrophotographic photoreceptor, inorganic photoconductive materials such as selenium, cadmium sulfide, zinc oxide, silicon, etc., are known, and these materials have been widely studied and practically used. The above inorganic materials have advantages and have various disadvantages at the same time. For example, selenium has drawbacks that it has difficulties in production conditions and is liable to undergo crystallization under heat or mechanical impact. Cadmium sulfide and zinc oxide have difficulties with respect of humidity resistance and durability. It is pointed out that silicon is insufficient in chargeability and causes difficulties in production. Further, selenium and cadmium sulfide also have a toxicity problem.
In contrast, organic photoconductive materials have advantages that they have good film formability, excellent flexibility, a light weight and good transparency, and that they permit easy designing of an electrophotographic photoreceptor to a broad wavelength region on the basis of a proper sensitizing method. For this reason, it is gradually attracting attention to put them into practical use.
Meanwhile, a photoreceptor for use in electrophotography is generally required to have the following properties as basic properties. That is, (1) it is to have high chargeability to corona discharge in a dark place, (2) the leakage of an obtained charge in a dark place (dark decay) is to be small, (3) the neutralization of the charge under irradiation with light (light decay) is to be rapid, and (4) the residual charge after the irradiation with light is to be small.
As organic photoconductive materials, photoconductive polymers including polyvinyl carbazole have been so far studied in various ways. However, these polymers are not necessarily sufficient in film formability, flexibility and adhesion, and it cannot be said that they fully have the above basic properties for a photoreceptor.
Concerning organic low-molecular-weight photoconductive compounds, it is possible to obtain a photoreceptor excellent in film formability, adhesion and mechanical strength such as flexibility when a binder and other materials are selected for forming a photoreceptor. However, it is difficult to find a compound suitable for maintaining a high-sensitivity characteristic.
For overcoming the above problems, there has been developed an organic photoreceptor imparted with the property of higher sensitivity by attaining the charge-generating function and the charge-transporting function separately with different substances. The characteristic feature of the above photoreceptor called a separated-function type is that materials suitable for the different functions can be selected from a broad range of materials, and since photoreceptors having performances as required can be easily produced, many materials have been studied.
Of these materials, a variety of substances such as phthalocyanine, a squarinium dyestuff, an azo pigment, a perylene pigment, etc., have been studied. Above all, an azo pigment has been studied and increasingly put to practical use since it can have a variety of molecular structures and can be expected to attain high charge generation efficiency. In the azo pigment, however, the relationship between its molecular structure and charge generation efficiency has not been clarified. The fact is that sutides on chemical synthesis have been and are being extensively made to search for optimum structures.
In recent years, a laser beam printer that uses laser beam in place of white light as a light source and is advantageous in high speed, high image qualities and non-impact has come to be popular together with the advance of information processing systems, and it is desired to develop materials that can comply with requirements therefor. In recent years, particularly, a semiconductor laser whose application to a compact disk, an optical disk, etc., has increased and whose technical progress is notable has been applied aggressively to the field of printers as a compact and highly reliable light source material. Since the above light source emits a beam having a wavelength of approximately 780 nm, a photoreceptor having the property of high-sensitivity to long-wavelength light around 780 nm is suitable, so that it is strongly desired to develop such photoreceptors. Under the circumstances, attempts are being actively made to develop photoreceptors using phthalocyanine having light absorption in a near infrared region, while no fully satisfactory photoreceptors have been developed.
Meanwhile, substances that work to transport charges include a hole-transporting substance and an electron-transporting substance. Various substances such as hydrazone compounds, stilbene compounds, etc., as a hole-transporting substance and 2,4,7-trinitro-9-fluorenone, diphenoxy derivatives, etc., as an electron-transporting substance are studied and practically used. With respect to these substances, however, the fact is that studies on chemical synthesis have been and are being increasingly made to search for optimum structures. In fact, many improvements have been made, but there have been obtained no substances that can fully satisfy the above basic properties or requirements such as high durability, etc., of a photoreceptor.
For practical use of an electrophotographic photoreceptor, the stability thereof in repeated use is also an unavoidable requirement. When a photoreceptor is repeatedly used, there come to appear various phenomena that cause image defects (ground fogging, black dots, white dots of images, etc.), such as a decrease in chargeabiity, a decrease in sensitivity and an increase in residual charge. For suppressing the above phenomena, various substances to be added to a photosensitive layer have been studied. However, the effect thereof varies depending upon kinds and combinations of a charge-generating substance and a charge-transporting substance, and under the circumstances, it is difficult to find a constitution sufficient for practical use.
Further, it is known that many properties required of a function-separation type photoreceptor, such as electric properties including chargeability, sensitivity, residual potential, pre-exposure property and repetitiveness and mechanical properties including mechanical strength such as scratch resistance and abrasion resistance are greatly influenced by types of binders used for dispersing or compatibilizing components such as a charge-generating substance and a charge-transporting substance. For constituting a photoreceptor to satisfy required properties, a number of combinations of components and binders have been proposed, while no sufficient combination has been obtained.
As described above, various improvements have been attempted for producing an electrophotographic photoreceptor. However, the fact is that there has been obtained no electrophotographic photoreceptor that fully satisfies the above basic properties required of an electrophotographic photoreceptor and the above requirements of high durability, and the like.
Under the circumstances, it is a first object of the present invention to provide an organic photoconductive material that gives an electrophotographic photoreceptor having high sensitivity and capability of exhibiting stabilized performances in repeated use when used in a photosensitive layer of an electrophotographic photoreceptor, and which can be also used in a sensor material, an electrostatic recording device, an optical switching element and various display devices such as an EL device and an electronic paper.
It is a second object of the present invention to provide an electrophotographic photoreceptor which exhibits a high charge potential and has high sensitivity and further which shows little or almost no change in various properties in repeated use and can exhibit stable performances.
The present inventors have made diligent studies for achieving the above objects. As a result, it has been found that the first object can be achieved by using an organic photoconductive material comprising a compound having a specific structure and that the second object can be achieved by an electrophotographic photoreceptor having a photosensitive layer containing the above organic photoconductive material. The present invention has been completed on the basis of the above findings.
That is, the present invention provides:
(1) an organic photoconductive material of, the general formula (1), 
wherein R1 is alkyl, aryl or a heterocyclic group, each of R2 and R3 is a hydrogen atom, a halogen atom or alkyl, each of Ar1 to Ar4 is aryl or a heterocyclic group and m is 1 when R1 is alkyl or 0 when R1 is aryl or a heterocyclic group, and
(2) an electrophotographic photoreceptor comprising a photosensitive layer containing a photoconductive material of the above general formula (1) on an electrically conductive substrate.